The present invention relates to transmission of control information in wireless communication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method for controlling transmission of Uplink Control Information (UCI) in a cellular wireless network with Carrier Aggregation (CA).
In a wireless communication system (e.g., a Long Term Evolution (LTE) fourth generation (4G) cellular network), a base station (e.g., an evolved Node-B or eNodeB (eNB) or a similar entity) may communicate with a mobile handset or User Equipment (UE) via uplink (UL) and downlink (DL) signaling over a radio frame. FIG. 1 illustrates an LTE radio frame 10 (Frame N) in a sequence of radio frames (Frames N−1, N, N+1, etc.) that may constitute the communication “link” between a base station and a mobile handset in a cellular network. The radio frame 10 may be of a fixed duration and may be divided into a fixed number of equally-sized subframes 12 identified as subframes “S0” through “S9” in FIG. 1. For example, in case of an LTE network, each radio frame 10 (i.e., each of Frame N, Frame N+1, etc.) may be of 10 ms duration, and may contain 10 subframes of 1 ms each as shown in FIG. 1. The frequency bandwidth of the radio frame 10 may depend on the overall system bandwidth available in the carrier network. Each subframe 12 in the radio frame 10 can be allocated as a DL subframe, as a UL subframe, or as a special subframe which consists of the Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS) fields (not shown). The GP field in the special subframe enables switching between downlink and uplink transmissions in a TDD system. Each subframe 12 contains information in the time domain as well as in the frequency domain (involving different sub-carriers).
A base station may transmit wireless channel resource allocation information to a mobile handset, terminal or User Equipment (UE) via a downlink control signal, such as the Physical Downlink Control Channel (PDCCH) signal in Third Generation Partnership Project (3GPP) 3G and 4G networks. Modern cellular networks (e.g., LTE) use Hybrid Automatic Repeat Request (HARQ) in which, after receiving this PDCCH downlink transmission (i.e., transmission from a base station to a mobile device) in a subframe, the UE may attempt to decode it and report to the base station whether the decoding was successful (ACK or Acknowledge) or not (NACK or Negative Acknowledge). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
Such reporting may be performed by the UE using uplink control signaling (i.e., transmission from a mobile device to a base station in a cellular network), which can include one or more of the following: (i) Hybrid-ARQ (HARQ) acknowledgements (ACK/NACK) for received downlink data (from the base station); (ii) terminal reports (e.g., in the form of one or more Channel Quality Indicator (CQI) bits) related to the downlink channel conditions. Such reports may be used by the base station to assist it in future downlink scheduling of the mobile handset; and (iii) scheduling requests by the UE, indicating that the mobile terminal or UE needs uplink resources for uplink data transmissions.
There are two different cases for transmitting uplink control signaling and which of these two cases to use depends on whether the terminal (i.e., the mobile handset or UE) is simultaneously transmitting data in the uplink (along with the control information): (1) In case the terminal does not transmit data at the same time as control information, control signaling is transmitted on the Physical Uplink Control Channel (PUCCH) in the 4G networks. The radio resource to be used for control channel transmissions is either indicated by the downlink transmission (from the base station) or is semi-statically configured by the base station. (2) In case the terminal needs to simultaneously transmit uplink control information and data, control and data are multiplexed prior to transmission and transmitted on the Physical Uplink Shared Channel (PUSCH) in the 3G and 4G networks.
Thus, if a mobile terminal has been assigned an uplink resource for data transmission and, at the same time instance, if the terminal has control information to transmit as well, the terminal will transmit the control information together with the data on PUSCH. Thus, when PUSCH is transmitted, any control signaling is multiplexed with data to maintain single carrier structure. However, in the absence of PUSCH, control signaling is on the PUCCH. The control information—known as the Uplink Control Information (UCI)—can consist of one or more of the following: (i) ACK/NACK feedback for the downlink transmission from the base station corresponding to and preceding the uplink transmission (PUCCH or PUSCH) from the UE carrying the UCI; (ii) a Channel Quality Indicator (CQI) indicating channel quality of the wireless communication channel between the base station and the UE; (iii) a Precoding Matrix Indicator (PMI) indicating a preferred precoding matrix for the control signaling (PUCCH or PUSCH); and (iv) a Rank Indicator (RI) indicating the number of useful transmission layers for the control channel (PUCCH or PUSCH) as experienced by the UE. The CQI, PMI, and RI parameters may constitute Channel Status Information (CSI). The CQI/PMI/RI reports (i.e., CSI reports) can be periodic on PUCCH, but can be smaller and often non-frequency-selective. Whereas, CQI/PMI/RI reports can be aperiodic on PUSCH, but may be frequency-selective and larger (wideband or UE-selected sub-band). The CSI report (with or without PMI depending on the UE's configured transmission mode) from the UE may be triggered by 1 bit in a PDCCH message from the base station.
The general operations of the LTE physical channels are described in various Evolved Universal Terrestrial Radio Access (E-UTRA) specifications such as, for example, 3GPP's Technical Specifications (TS) 36.201 (“Physical Layer: General Description”), 36.211 (“Physical Channels and Modulation”), 36.212 (“Multiplexing and Channel Coding”), 36.213 (“Physical Layer Procedures”), 36.214 (“Physical Layer—Measurements”), and 36.331 (“Radio Resource Control (RRC)—Protocol Specification”). These specifications may be consulted for additional reference and are incorporated herein by reference.
It is observed here that LTE Release-8 (Rel-8) now has been standardized to support operating bandwidths of up to 20 MHz. However, in order to meet International Mobile Telecommunications (IMT)-Advanced requirements, 3GPP has initiated work on LTE Release-10 (Rel-10) (“LTE Advanced”) to support bandwidths larger than 20 MHz. One important requirement in LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This includes spectrum compatibility, i.e., an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of (smaller) LTE carriers to an LTE Rel-8 terminal (i.e., mobile handset or UE). Each such smaller carrier can be referred to as a Component Carrier (CC). It is observed here that during initial deployments of LTE Rel-10, the number of LTE Rel-10-capable terminals may be smaller compared to many LTE legacy terminals (e.g., Rel-8 or Rel-9 terminals). Therefore, it is necessary to assure an efficient use of a wide (Rel-10) carrier also for legacy terminals. In other words, it should be possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. One way to obtain this efficient usage is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where each CC has, or at least the possibility to have, the same structure as a Rel-8 carrier. FIG. 2 illustrates the principle of CC aggregation. As shown in FIG. 2, an operating bandwidth of 100 MHz (indicated by reference numeral “14”) in Rel-10 may be constructed by the aggregation of five (contiguous, for simplicity) smaller bandwidths of 20 MHz (in compliance with Rel-8 requirements) as indicated by reference numerals “16” through “20”. It is noted here that Rel-10 supports aggregation of up to five carriers, each with a bandwidth of up to 20 MHz. Thus, for example, if desired, carrier aggregation in Rel-10 also may be used to aggregate two carriers of 5 MHz bandwidth each. The carrier aggregation in uplink and downlink may thus support higher data rates than possible in legacy communication systems (i.e., UE's operating under 3GPP Rel-8, Rel-9, or below). UE's capable of operating only over a single Downlink/Uplink (DL/UL) pair may be referred to as “Legacy UE's”, whereas UE's capable of operating over multiple DL/UL CCs may be referred to as “Advanced-UE's”.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A “symmetric configuration” refers to the case where the number of CCs in downlink and uplink is the same, whereas an “asymmetric configuration” refers to the case where the number of CCs is different in uplink and downlink. It is important to note that the number of CCs configured in the network may be different from the number of CCs “seen” by a terminal (or UE): A terminal may, for example, support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs. The link between DL CCs and UL CCs can be UE-specific.
Scheduling of a CC (e.g., grant of radio resources for UL transmission from a UE on the CC) is done on the PDCCH via downlink assignments (from the base station). In Rel-8, a terminal only operates with one DL and one UL CC. Therefore, the association between DL assignment/UL grant and the corresponding DL and UL CCs is clear in Rel-8. However, in Rel-10, cross-carrier scheduling may be enabled where the PDCCH containing DL assignment/UL grant is transmitted on a CC that is different from the CC on which the Physical Downlink Shared Channel (PDSCH) or its associated PUSCH are transmitted.